1. Field of the Invention
The present invention relates to a windshield wiper for an automobile in which a wiping radius is varied by using a four-link linkage so that a far corner of a windshield can be cleaned by a wiper blade.
2. Description of Related Art
A windshield wiper for an automobile usually has a constant wiping radius because its pivotal center is fixed. The far corner of a windshield cannot be cleaned by the usual windshield wiper though it is desired to wipe a large area to secure a good view for a driver. A wiper for cleaning the far corner of the windshield has been proposed, for example, in JP-A-61-181745. A five-link linkage mechanism is used in the proposed wiper to enlarge the wiping radius at the far corner of the windshield so that the far corner can be also cleaned by a wiper blade.
FIG. 15 shows a similar kind of a conventional wiper in which a four-link linkage is used. Wiper arm 51 including retainer 57 is pivotally supported by first link 54 and second link 55. Wiper blade 56 for wiping the windshield is supported at the top end of wiper arm 51. First link 54 is pivotally supported by first axis 52 at its one end, and second link 55 is also pivotally supported by second axis 53 at its one end. Both first and second axes 52, 53 are fixed to a vehicle frame and constitutes a stationary link 100. Four-link linkage 58 is constituted by wiper arm 51, first link 54, second link 55 and stationary link 100. Actuator lever 64 is rigidly fixed to first link 54 to form a single straight link pivotally supported by first axis 52. Actuating lever 64 is driven by a drive mechanism. The drive mechanism incudes motor 59, worm 60 rotated by motor 59, worm wheel 61 engaging worm 60, crank lever 62 driven by worm wheel 61, and link rod 63 connecting crank lever 62 to actuator lever 64. Actuator lever 64 is pivotally rotated around first axis 52, and thereby first link 54 fixed to actuator lever 64 is pivotally driven around first axis 52. Thus, wiper arm 51 is driven back and forth between tow positions, a bottom position (a home position) and an upper position. During one rotation of worm wheel 61, wiper arm 51 makes one round trip from the bottom position to the upper position and from the upper position back to the bottom. The wiping radius of the wiper becomes maximum at an interim position between the bottom and upper positions, thereby wiping the far corner of the windshield.
This kind of wiper is usually used in a large size vehicle, and it is bulky and heavy because of the four-link linkage, compared with a wiper having a constant wiping radius. Also, a higher driving force is required especially when the moving direction of the wiper is switched at a turning position, because power transmission efficiency of the drive mechanism becomes low at the turning position. FIG. 16 shows motor load which is required near the bottom position. The left graph shows motor load A required when the wiper ascends from the bottom position, and the right graph shows motor load B required when the wiper descends toward the bottom position. When the wiper arm makes a turn and ascends upward from the bottom position, the force required to lift up the wiper arm (which is equal to motor load A) is: Fa+Fg+Fm, where Fa is a force for accelerating the wiper arm at a predetermined acceleration, Fg is a gravitational force due to the weight of the wiper arm and the links, and Fm is a force due to friction at axes and pivotal joints. On the other hand, the force required when the wiper arm is descending toward the bottom position (which is equal to load B) is: Fb+Fm-Fg, where Fb is the force for driving the wiper arm downward toward the bottom position. Gravitational force Fg is added when the wiper arm is ascending while it is subtracted when the wiper arm is descending. Therefore, the highest motor load is required when the wiper is ascending from the bottom position.
For the reason mentioned above, a large motor which is able to cover the highest load A has to be used in the conventional wiper having the variable wiping radius.